When you no longer care about doing permanent damage to your career. An exception may be when he's committing some fireable offense - sexual harassment, drug or alcohol abuse, a snub-nosed .38 strapped to his ankle. You may want to loop in HR first so you have an ally. Or go over his head if you are so secure in your relationship with his bosses that you feel confident you can move with impunity. But that support can disappear quickly. These guys tend to stick together. And remember the old saying: When you aim for the king, you better not miss.